1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers. More specifically, it relates to vertical cavity surface emitting lasers having integrally packaged power monitors.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer surface. VCSELs can be formed from a wide range of material systems to produce specific characteristics. VCSELs typically have active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 is disposed with an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAS substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20 with quantum wells is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that is formed by implanting protons into the top mirror stack 24 or by forming an oxide layer. The insulating region 40 has a conductive annular central opening 42. Thus, the central opening 42 forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 flow through the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, providing a p-type substrate), different material systems can be used, operational details can be varied, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have are not without problems. For example, it is sometimes important to control the optical power out of a VCSEL. In many applications the desired optical power output is the highest value possible, consistent with eye safety and reliability. Ideally, the desired optical power output is achieved despite manufacturing variances and tolerances, temperature effects, and aging. It is known to sense the optical power output and to use electronic circuitry to control that output. Ideally, a VCSEL and an output power sensor are designed to work together efficiently. One way of doing this is illustrated in U.S. Pat. No. 6,069,905. That patent discloses a mirror-based laser intensity control system in which a VCSEL and an output power sensor are situated on one substrate. However, at least because of the mirror, that technique might not be optimal.
Therefore, a new technique of integrating a VCSEL and an output power sensor would be beneficial. Even more beneficial would be a technique in which an output power sensor is directly aligned with a VCSEL. Still more beneficial would be a technique of integrally packaging an output power sensor and a VCSEL such that those elements are optically aligned.